Two-group zoom lens

ABSTRACT

In a compact two-group zoom lens composed of a positive front group (G 1 ) and a negative rear group (G 2 ), the rear group (G 2 ) is constituted by negative, positive, and negative lenses, thereby improving productivity while attaining a high magnification of about 3×. The zoom lens is constituted by the front group (G 1 ) comprising four sheets of lenses (L 1  to L 4 ) and the rear group (G 2 ) comprising three sheets of lenses (L 5  to L 7 ). The rear group (G 2 ) has the fifth lens (L 5 ) made of a biconcave plastic lens having aspheric surfaces on both sides, whose surface with a stronger curvature is directed onto the object side on the optical axis (X); the sixth lens (L 6 ) made of a biconvex lens having a surface with a stronger curvature directed onto the object side; and the seventh lens (L 7 ) made of a negative meniscus lens having a concave surface directed onto the object side.

RELATED APPLICATIONS

This application claims the priority of Japanese Patent Application No.7-313628 filed on Nov. 6, 1995, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a compact two-group zoom lens having avariable power ratio of about 3, which is suitable for still cameras andelectronic still cameras.

2. Description of the Prior Art

Conventionally, a zoom lens composed of two groups which arerespectively positive and negative has widely been used for compactcameras. Recently, of such a lens, that having a variable power ratio ofabout 3 has been developed in response to a demand for highermagnification in compact cameras.

This positive and negative two-group zoom lens, which is advantageous inthat the whole lens length can be shortened while having a zoomfunction, is particularly suitable for compact cameras.

Of the above-mentioned positive and negative two-group zoom lens, therehas been known the one in which the lens closest to the object in therear group having a negative refractive power as a whole is constitutedby a positive lens, for example, as disclosed in Japanese UnexaminedPatent Publication No. 5-188293 and No. 6-3591.

In the lens having such a configuration, however, since each of thefront group having a positive refractive power as a whole and the firstlens (closest to the front group) in the rear group has a positiverefractive power, the total positive refractive power of the zoom lens,in the one which has not yet attained a high magnification and compactsize in particular, has become very strong, thereby yielding a greaterdifference in imaging magnification with respect to the negative lenssystem of the rear group. Accordingly, it has been difficult to keepfluctuation in aberration and accuracy in eccentricity at favorablelevels, whereby productivity has been deteriorated.

While a lens system in which the rear group is constituted by twonegative lens sheets as disclosed in Japanese Patent Publication No.6-25825 has been known, such a lens system has a low variable powerratio of about 1.5 due to its insufficient achromatic effect though withan improved imaging relationship.

SUMMARY OF THE INVENTION

In view of such a circumstance, the object of the present invention isto provide a two-group zoom lens which is excellent in productivity andinexpensive to manufacture while satisfying demands for highermagnification and compact size.

In the two-group zoom lens of the present invention, a rear group G₂having a negative refractive power is constituted by a negative lens L₁,a positive lens L₂, and a negative lens L₃ successively disposed fromthe object side, thereby overcoming the problems of the conventionalpositive and negative two-group zoom lens.

Namely, the two-group zoom lens of the present invention is a two-groupzoom lens comprising, successively from the object side, a front groupG₁ having a positive refractive power and a rear group G₂ having anegative refractive power, in which distance between the front group andthe rear group is changed so as to alter the focal length of the wholelens system;

wherein the rear group G₂ comprises, successively from the object side,a first lens L_(B1) having a negative refractive power, a second lensL_(B2) having a positive refractive power, and a negative third lensL_(B3) having a concave surface directed onto the object side.

Assuming that paraxial focal length of the first lens L_(B1) is f_(B1),paraxial focal length of the second lens L_(B2) is f_(B2), focal lengthof the whole lens system at its wide angle end is f_(w), and Abbe numberof glass material of the second lens L_(B2) is υ_(B2), such a two-groupzoom lens is desirably configured so as to satisfy the followingconditional expressions (1) to (3):

    (1) -1.0<f.sub.w /f.sub.B1 <-0.1

    (2) 0.5<f.sub.w /f.sub.B2 <1.7

    (3) υ.sub.B2 <43.0

Further, desirably, the zoom lens is configured such that at least oneaspherical lens is disposed in each lens group or such that at least oneplastic lens is disposed in each lens group.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a basic lens configuration (at thewide angle end) in accordance with Embodiment 1 of the presentinvention;

FIG. 2 is a schematic view showing a basic lens configuration (at thewide angle end) in accordance with Embodiment 2 of the presentinvention;

FIG. 3 is a schematic view showing a basic lens configuration (at thewide angle end) in accordance with Embodiment 3 of the presentinvention;

FIG. 4 is an aberration chart of the lens in accordance with Embodiment1 at its wide angle end;

FIG. 5 is an aberration chart of the lens in accordance with Embodiment1 at its middle region;

FIG. 6 is an aberration chart of the lens in accordance with Embodiment1 at its telephoto end;

FIG. 7 is an aberration chart of the lens in accordance with Embodiment2 at its wide angle end;

FIG. 8 is an aberration chart of the lens in accordance with Embodiment2 at its middle region;

FIG. 9 is an aberration chart of the lens in accordance with Embodiment2 at its telephoto end;

FIG. 10 is an aberration chart of the lens in accordance with Embodiment3 at its wide angle end;

FIG. 11 is an aberration chart of the lens in accordance with Embodiment3 at its middle region; and

FIG. 12 is an aberration chart of the lens in accordance with Embodiment3 at its telephoto end.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of the present invention will be explainedwith reference to drawings.

FIGS. 1 to 3 show compact two-group zoom lenses in accordance withrespective embodiments. Namely, each of them is a zoom lens comprising,successively from the object side, a front group G₁ having a positiverefractive power as a whole and a rear group G₂ having a negativerefractive power as a whole; in which, when zooming, the first lensgroup G₁ and the second lens group G₂ move along an optical axis X (withlower part of each drawing indicating the locus of movement of eachgroup) while changing the distance therebetween, thereby changing focallength f of the whole system and efficiently converging a luminous fluxonto an imaging surface (whereby a parallel luminous flux incident onthe lens system in parallel to the optical axis X forms an image at animaging position P).

Also, the rear group G₂ is configured such that, successively from theobject side, a rear-group first lens L_(B1) having a negative refractivepower, a rear-group second lens L_(B2) having a positive refractivepower, and a negative rear-group third lens L_(B3) having a concavesurface directed onto the object side are disposed therein.

Further, assuming that paraxial focal length of the first lens L_(B1) isf_(B1), paraxial focal length of the second lens L_(B2) is f_(B2), focallength of the whole lens system at its wide angle end is fw, and Abbenumber of glass material of the second lens L_(B2) is υ_(B2), thefollowing conditional expressions (1) to (3):

    (1) -1.0<f.sub.w /f.sub.B1 <-0.1

    (2) 0.5<f.sub.w /f.sub.B2 <1.7

    (3) υ.sub.B2 <43.0

are satisfied.

Also, the zoom lens of each embodiment mentioned above is configuredsuch that at least one aspherical lens is disposed in each of the firstgroup G₁ and the second group G₂ and such that at least one plastic lensis disposed in each of the first group G₁ and the second group G₂.

As mentioned above, in the two-group zoom lens of each embodiment, therear group G₂ having a negative refractive power comprises, successivelyfrom the object side, the negative lens L_(B1), the positive lensL_(B2), and the negative lens L_(B3), so as to overcome the problems ofthe conventional positive and negative two-group zoom lens, whereby alens system which has a favorable productivity while being ahigh-magnification zoom lens with a magnification of about 3× can beconstituted.

In the following, technical significance of each of the above-mentionedexpressions (1) to (3) will be explained.

Namely, the above-mentioned conditional expression (1) defines therefractive power of the lens L_(B1) closest to the object side in therear group G₂. Below the lower limit of expression (1), the whole lengthof the lens system becomes too large to attain a compact size. Beyondthe upper limit, by contrast, though the whole length of the lens systemis favorably shortened so as to attain a compact size, it is undesirablein that a strict accuracy is required for the relationship between thelenses L_(B2) and L_(B3) in the subsequent step.

Also, the above-mentioned conditional expression (2) defines therefractive power of the positive lens L_(B2) constituting the rear groupG₂. Below the lower limit of expression (2), the whole length of thelens system becomes too large to attain a compact size, whilecorrections for chromatic aberration and distortion are deteriorated.Beyond the upper limit, by contrast, though it is advantageous in termsof compactness, problems such as requirement for higher precision inparts may occur.

Further, the above-mentioned conditional expression (3) relates to Abbenumber υ_(B2) of glass material of the second lens L_(B2) constitutingthe rear group G₂, defining a range which satisfies conditions for therefractive power and achromatism of the lens L_(B2) in the rear groupG₂. Beyond the upper limit thereof, the refractive power of the lensL_(B2) becomes too strong in order to correct chromatic aberration,thereby making it necessary to attain a higher precision in parts.

Also, when an aspherical lens is disposed in each of the lens groups G₁and G₂ as mentioned above, the deterioration in aberration due to ahigher magnification can be corrected without increasing the number ofsheets constituting the lens.

Further, when at least one plastic lens is disposed in each of the lensgroups G₁ and G₂ as mentioned above, the cost for making the zoom lenscan be lowered. Also, when this plastic lens is made aspheric, the costcan be further lowered. When the plastic lenses respectively disposed inthe lens groups G₁ and G₂ are set to have refractive powers having signsof polarity opposite to each other, the fluctuation in imaging positiondue to change in temperature can be minimized, thereby compensating fora shortcoming of a plastic lens that it is sensitive to change intemperature.

In the following, each embodiment will be explained in detail.

(Embodiment 1)

As shown in FIG. 1, the two-group zoom lens of Embodiment 1 isconstituted by the front group G₁ comprising four sheets of lenses L₁ toL₄ and the rear group G₂ comprising three sheets of lenses L₅ to L₇. Thefront group G₁ has the first lens L₁ made of a negative meniscus lenshaving a concave surface directed onto the object side; the second lensL₂ made of a biconvex plastic lens having an aspheric surface on itssurface on the object side; the third lens L₃ made of a negativemeniscus lens having a concave surface directed onto the imaging surfaceside; and the fourth lens L₄ made of a biconvex lens having a surfacewith a stronger curvature directed onto the imaging surface side. Therear group G₂ has the fifth lens L₅ made of a biconcave plastic lenshaving aspheric surfaces on both sides, whose surface with a strongercurvature is directed onto the object side on the optical axis X; thesixth lens L₆ made of a biconvex lens having a surface with a strongercurvature directed onto the object side; and the seventh lens L₇ made ofa negative meniscus lens having a concave surface directed onto theobject side.

Next, radius of curvature R (mm) of each lens surface, center thicknessof each lens and air gap between neighboring lenses (collectivelyreferred to as "axial spacing" hereinafter) D (mm), refractive power Nand Abbe number υ of each lens at d line in the two-group zoom lens inaccordance with Embodiment 1 are shown in Table 1.

Here, the numbers in this table (as well as in Tables 3 and 5)successively increase from the object side.

Also, each of the surfaces designated by "*" on the right side of Rvalue in Table 1 (as well as in Tables 3 and 5) is formed as an asphericsurface and refers to an aspherical form computed by the followingexpression (A):

    Z=CY.sup.2 / 1+(1-KC.sup.2 Y.sup.2).sup.1/2 !+A.sub.1 Y.sup.4 +A.sub.2 Y.sup.6 +A.sub.3 Y.sup.8 +A.sub.4 Y.sup.10                (A)

wherein Z is the length (mm) of a perpendicular drawn from a point onthe aspheric surface having a height Y from the optical axis to atangential plane (plane perpendicular to the optical axis) of the apexof the aspheric surface; C is the paraxial curvature of the asphericsurface; Y is the height (mm) from the optical axis; K is theeccentricity; and A₁ to A₄ are aspherical coefficients of the fourth,sixth, eighth, and tenth orders, respectively.

Also, the upper part of Table 2 shows values of the respective constantsC, K, and A₁ to A₄ of each aspheric surface indicated by theabove-mentioned expression (A).

Further, the middle part of Table 2 shows focal length f', back focuslength l', F number, and field angle 2ω of the whole lens system as wellas the distance between the two lens groups G₁ and G₂ at each of thepositions of wide angle end (W), middle region (M), and telephoto end(T).

Also, focal length f₁ of the front group G₁, focal length f₂ of the reargroup G₂, variable power ratio, paraxial focal length f_(B1) of thefirst lens (fifth lens L₅) in the rear group G₂, and paraxial focallength f_(B2) of the second lens (sixth lens L₆) in the rear group G₂are set as shown in the lower part of Table 2.

Further, f_(w) /f_(B1) =-0.6248 and f_(w) /f_(B2) =1.1103 as shown inthe lower part of Table 2, while υ_(B2) (Abbe number of the sixth lensL₆)=35.5 as shown in Table 1, thereby satisfying all of theabove-mentioned expressions (1) to (3).

(Embodiment 2)

As shown in FIG. 2, the two-group zoom lens of Embodiment 2 is similarto that of Embodiment 1 in that it is constituted by the front group G1comprising four sheets of lenses L₁ to L₄ and the rear group G₂comprising three sheets of lenses L₅ to L₇. This zoom lens is differentfrom that of Embodiment 1, however, in that the second lens L₂ is abiconvex lens having a surface with a stronger curvature directed ontothe imaging surface side, that the fifth lens L₅ is a negative meniscuslens whose surface on the object side alone is an aspheric surface, andthat the sixth lens L₆ is a biconvex lens having a surface with astronger curvature directed onto the imaging surface side.

Next, radius of curvature R (mm) of each lens surface, axial spacing D(mm), refractive power N and Abbe number υ of each lens at d line in thetwo-group zoom lens in accordance with Embodiment 2 are shown in Table3.

Also, the upper part of Table 4 shows values of the respective constantsC, K, and A₁ to A₄ of each aspheric surface indicated by theabove-mentioned expression (A).

Further, the middle part of Table 4 shows focal length f', back focuslength l', F number, and field angle 2ω of the whole lens system as wellas the distance between the two lens groups G₁ and G₂ at each of thepositions of wide angle end (W), middle region (M), and telephoto end(T).

Also, focal length f₁ of the front group G₁, focal length f₂ of the reargroup G₂, variable power ratio, paraxial focal length f_(B1) of thefirst lens (fifth lens L₅) in the rear group G₂, and paraxial focallength f_(B2) of the second lens (sixth lens L₆) in the rear group G₂are set as shown in the lower part of Table 4.

Further, f_(w) /f_(B1) =-0.4356 and f_(w) /f_(B2) =0.6567 as shown inthe lower part of Table 4, while υ_(B2) (Abbe number of the sixth lensL₆)=31.2 as shown in Table 3, thereby satisfying all of theabove-mentioned expressions (1) to (3).

(Embodiment 3)

As shown in FIG. 3, the two-group zoom lens of Embodiment 3 isconstituted by the front group G₁ comprising five sheets of lenses L₁ toL₅ and the rear group G₂ comprising three sheets of lenses L₆ to L₈. Thefront group G₁ has the first lens L₁ made of a biconcave lens having asurface with a stronger curvature directed onto the object side, thesecond lens L₂ made of a biconvex lens having a surface with a strongercurvature directed onto the object side, the third lens L₃ (cemented tothe second lens L₂) made of a biconcave lens having a surface with astronger curvature directed onto the object side, the fourth lens L₄made of a biconvex lens having a surface with a stronger curvaturedirected onto the object side, and the fifth lens L₅ made of a biconvexlens having a surface with a stronger curvature directed onto theimaging surface side. The rear group G₂ has the sixth lens L₆ made of anegative plastic lens having aspheric surfaces on both sides, whosesurface with a stronger curvature is directed onto the object side onthe optical axis X; the seventh lens L₇ made of a biconvex lens having asurface with a stronger curvature directed onto the object side; and theeighth lens L₈ made of a biconcave lens having a surface with a strongercurvature directed onto the object side.

Next, radius of curvature R (mm) of each lens surface, axial spacing D(mm), refractive power N and Abbe number υ of each lens at d line in thetwo-group zoom lens in accordance with Embodiment 3 are shown in Table5.

Also, the upper part of Table 6 shows values of the respective constantsC, K, and A₁ to A₄ of each aspheric surface indicated by theabove-mentioned expression (A).

Further, the middle part of Table 6 shows focal length f', back focuslength l', F number, and field angle 2ω of the whole lens system as wellas the distance between the two lens groups G₁ and G₂ at each of thepositions of wide angle end (W), middle region (M), and telephoto end(T).

Also, focal length f₁ of the front group G₁, focal length f₂ of the reargroup G₂, variable power ratio, paraxial focal length f_(B1) of thefirst lens (sixth lens L₆) in the rear group G₂, and paraxial focallength f_(B2) of the second lens (seventh lens L₇) in the rear group G₂are set as shown in the lower part of Table 6.

Further, f_(w) /f_(B1) =-0.7956 and f_(w) /f_(B2) =1.5293 as shown inthe lower part of Table 6, while ω_(B2) (Abbe number of the sixth lensL₆)=35.3 as shown in Table 5, thereby satisfying all of theabove-mentioned expressions (1) to (3).

FIGS. 4 to 6, FIGS. 7 to 9, and FIGS. 10 to 12 are aberration chartsshowing various kinds of aberration (spherical aberration, fieldcurvature, and distortion) of the two-group zoom lenses respectively inEmbodiments 1 to 3 at their wide angle end, middle region, and telephotoend. As can be seen from these charts, in accordance with the zoomlenses of these embodiments, various kinds of aberration can befavorably corrected at any position from the wide angle end to thetelephoto end.

Without being restricted to the foregoing embodiment, in the compacttwo-group zoom lens of the present invention, for example, the numberand form of lenses constituting each lens group as well as the number ofaspherical lenses and plastic lenses and the form of the asphericsurface can be selected appropriately.

For example, the number of the lenses constituting the front group maybe three or five or more as long as the front group as a whole has apositive refractive power.

As explained in the foregoing, in the compact two-group zoom lens of thepresent invention, of the positive front group and the negative reargroup constituting the zoom lens, the rear group is constituted bynegative, positive, and negative lenses, whereby various kinds ofaberration can be made favorable while the condition for precision inlens parts is alleviated. Accordingly, while a high magnification ofabout 3× is attained, improvement in productivity and reduction inmanufacturing cost can be achieved.

                  TABLE 1                                                         ______________________________________                                        Surface No.                                                                           R        D              N     υ                               ______________________________________                                        1       -13.309  1.10           1.62041                                                                             60.4                                    2       -21.895  0.50                                                         3       44.218*  2.00           1.49023                                                                             57.6                                    4       -65.081  0.85                                                         5       75.255   0.86           1.88300                                                                             40.8                                    6       16.068   0.19                                                         7       14.904   5.00           1.48749                                                                             70.2                                    8       -11.725  15.50 ˜ 9.50 ˜ 4.50                              9       36.855*  1.30           1.49023                                                                             57.6                                    10      189.549* 0.30                                                         11      34.684   3.20           1.59270                                                                             35.5                                    12      -51.099  3.60                                                         13      -10.743  1.30           1.71300                                                                             53.9                                    14      -101.739                                                              ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________    Aspheric surface                                                                      C    K     A.sub.1 A.sub.2 A.sub.3 A.sub.4                            __________________________________________________________________________    No. 3   0.022615                                                                           5.16025                                                                             -0.12216 × 10.sup.-3                                                            -0.89406 × 10.sup.-6                                                            0.94966 × 10.sup.-9                                                             -0.12275 × 10.sup.-9         No. 9   -0.027133                                                                          -17.87734                                                                           0.10805 × 10.sup.-3                                                               0.28167 × 10.sup.-5                                                           -0.73905 × 10.sup.-7                                                              0.28109 × 10.sup.-9         No. 10 0.005276                                                                           -6579.924                                                                           0.17510 × 10.sup.-3                                                             -0.22087 × 10.sup.-5                                                            0.10913 × 10.sup.-7                                                             -0.28647 × 10.sup.-9         __________________________________________________________________________                  W                M  T                                           __________________________________________________________________________           f'     39.25            62.71                                                                            124.95                                             1'     9.51             28.89                                                                            80.30                                              Fno    3.5              5.6                                                                              11.3                                               Field angle 2ω                                                                 57.7°     38.1°                                                                     19.6°                                       Distance                                                                             15.50            9.50                                                                             4.50                                        __________________________________________________________________________               f.sub.1 =                                                                          27.60                                                                           variable power ratio 3.19                                              f.sub.2 =                                                                        -22.80                                                                            f.sub.w /f.sub.B1 =                                                                            -0.6248                                               f.sub.B1 =                                                                       -62.82                                                                            f.sub.w /f.sub.B2 =                                                                              1.1103                                              f.sub.B2 =                                                                         35.35                                                         __________________________________________________________________________

                  TABLE 3                                                         ______________________________________                                        Surface No.                                                                           R        D              N     ν                                    ______________________________________                                        1       -22.703  2.09           1.6516                                                                              58.4                                    2       -97.255  2.10                                                         3       87.260*  2.00           1.49023                                                                             57.6                                    4       -80.287  0.55                                                         5       36.938   2.28           1.83500                                                                             43.0                                    6       14.139   0.19                                                         7       13.541   5.71           1.48749                                                                             70.4                                    8       -12.032  16.00 ˜ 10.00 ˜ 5.00                             9       368.140* 1.30           1.49023                                                                             57.6                                    10      39.615   0.70                                                         11      7735.94  3.00           1.68893                                                                             31.2                                    12      -41.655  4.18                                                         13      -10.678  1.30           1.71300                                                                             53.9                                    14      -42.258                                                               ______________________________________                                    

                                      TABLE 4                                     __________________________________________________________________________    Aspheric surface                                                                      C    K    A.sub.1 A.sub.2 A.sub.3 A.sub.4                             __________________________________________________________________________    No. 3   0.011460                                                                           -21.8600                                                                           -0.11245 × 10.sup.-3                                                            -0.26858 × 10.sup.-5                                                            0.82360 × 10.sup.-7                                                             -0.13162 × 10.sup.-8          No. 9   0.002716                                                                           2050.0100                                                                            0.83565 × 10.sup.-4                                                           -0.92860 × 10.sup.-6                                                            0.26989 × 10.sup.-7                                                             -0.21888 × 10.sup.-9          __________________________________________________________________________                  W               M  T                                            __________________________________________________________________________           f'     39.50           63.69                                                                            130.03                                              1'     9.75            30.58                                                                            87.71                                               Fno    3.80            6.20                                                                             12.0                                                field angle 2ω                                                                 57.4°    37.5°                                                                     18.9°                                        Distance                                                                             16.00           10.00                                                                            5.00                                         __________________________________________________________________________               f.sub.1 =                                                                          26.92                                                                           variable power ratio 3.29                                              f.sub.2 =                                                                        -23.19                                                                            f.sub.w /f.sub.B1 =                                                                            -0.4356                                               f.sub.B1 =                                                                       -90.67                                                                            f.sub.w /f.sub.B2 =                                                                              0.6567                                              f.sub.B2 =                                                                         60.15                                                         __________________________________________________________________________

                  TABLE 5                                                         ______________________________________                                        Surface No.                                                                           R        D              N     υ                               ______________________________________                                        1       -19.335  1.68           1.62041                                                                             60.3                                    2       38.336   1.38                                                         3       13.375   4.16           1.48749                                                                             70.3                                    4       -14.742  1.18           1.80236                                                                             47.3                                    5       626.493  0.49                                                         6       47.611   2.02           1.49023                                                                             57.6                                    7       -73.366* 0.10                                                         8       45.184   3.60           1.48749                                                                             70.3                                    9       -14.488  15.20 ˜ 9.80 ˜ 4.30                              10      978.783* 1.40           1.49023                                                                             57.6                                    11      22.706*  0.20                                                         12      18.825   4.268          1.59270                                                                             35.3                                    13      -49.343  2.6485                                                       14      -11.781  1.30           1.80600                                                                             39.6                                    15      1570.958                                                              ______________________________________                                    

                                      TABLE 6                                     __________________________________________________________________________    Aspheric surface                                                                      C    K      A.sub.1 A.sub.2 A.sub.3 A.sub.4                           __________________________________________________________________________    No. 7   -0.01363                                                                           48.1840                                                                              0.17662 × 10.sup.-3                                                             0.22948 × 10.sup.-6                                                               0.17346 × 10.sup.-7                                                           -0.24432 × 10.sup.-9                                                    1                                 No. 10  -0.00102                                                                           -0.14189 × 10.sup.6                                                            0.11120 × 10.sup.-3                                                             0.13896 × 10.sup.-5                                                             -0.25173 × 10.sup.-7                                                            0.21179 × 10.sup.-9         No. 11    0.04404                                                                           4.83576                                                                             0.19922 × 10.sup.-4                                                             0.83870 × 10.sup.-7                                                             -0.11496 × 10.sup.-7                                                            .sup. 0.58056 ×                                                         10.sup.-10                        __________________________________________________________________________                   W                M  T                                          __________________________________________________________________________            f'     36.00            52.97                                                                            101.96                                             1'     10.61            26.24                                                                            71.32                                              Fno    3.8              5.6                                                                              10.8                                               Field angle 2ω                                                                 62.0°     44.4°                                                                     23.9°                                       Distance                                                                             15.20            9.80                                                                             4.30                                       __________________________________________________________________________               f.sub.1 =                                                                          25.67                                                                           variable power ratio 2.83                                              f.sub.2 =                                                                        -23.62                                                                            f.sub.w /f.sub.B1 =                                                                            -0.7956                                               f.sub.B1 =                                                                       -45.25                                                                            f.sub.w /f.sub.B2 =                                                                              1.5293                                              f.sub.B2 =                                                                         23.54                                                         __________________________________________________________________________

What is claimed is:
 1. A two-group zoom lens comprising, successivelyfrom an object side, a front group (G₁) having a positive refractivepower and a rear group (G₂) having a negative refractive power, in whichdistance between said front group and said rear group is changed so asto alter focal length of said zoom lens as a whole;wherein said reargroup (G₂) consists, successively from the object side, a first lens(L_(B1)) having a negative refractive power, a second lens (L_(B2))having a positive refractive power, and a negative third lens (L_(B3))having a concave surface directed onto the object side; and wherein thesecond lens L_(b2) in the rear group has a radius of curvature of asurface faces the first lens (L_(b1)) of the rear group which differsfrom a radius of curvature of the image-side surface of the first lens(L_(b1)).
 2. A two-group zoom lens according to claim 1, wherein, aparaxial focal length of said first lens (L_(B1)) is f_(B1), paraxialfocal length of said second lens (L_(B2)) is f_(B2), focal length ofsaid zoom lens as a whole at its wide angle end is f_(w), and Abbenumber of glass material of said second lens (L_(B2)) is ω_(B2), saidzoom lens satisfies the following conditional expressions (1) to (3):

    (1) -1.0<f.sub.w /f.sub.B1 <-0.1

    (2) 0.5<f.sub.w /f.sub.B2 <1.7

    (3) ω.sub.B2 <43.0


3. A two-group zoom lens according to claim 1, wherein at least oneaspherical lens is disposed in each of said front and rear lens groups(G₁) and (G₂); andwherein a lens closest to said object has onlyspherical lenses.
 4. A two-group zoom lens according to claim 1, whereinat least one plastic lens is disposed in each of said front and rearlens groups (G₁) and (G₂); andwherein a first lens of said front lensgroup (G₁) is a glass lens.
 5. A two-group zoom lens according to claim1, wherein at least one plastic lens is disposed in each of said frontand rear lens groups (G₁) and (G₂).